1. Field of the Invention
This invention relates to a process for producing a peroxide-vulcanizable, fluorine-containing elastomer, and more particularly to a process for producing a fluorine-containing elastomer having those halogen atoms in the molecule which can act as cross-linking points for peroxide vulcanization.
2. Description of the Prior Art
Generally, vulcanized fluorine-containing elastomers have distinguished heat resistance, solvent resistance, weathering and ozone resistances, creep resistance, etc., and are commercially widely used as a sealing material for oil seal, packing material, gasket, O ring, etc., or as a diaphragm material, a hose lining material, a coating material, an adhesive, etc.
One of the conventional processes for obtaining such a vulcanized, fluorine-containing elastomer is based on a peroxide vulcanization method using an organic peroxide as a vulcanizing agent, where fluorine-containing elastomers having iodine atoms or bromine atoms as cross-linking points are used. Some of the examples will be given below, and they have the following problems.
Japanese Patent Application Kokai (Laid-open) No. 53-125,491 discloses a process using an iodine compound represented by the general formula Rf Ix, wherein Rf represents a fluorohydrocarbon group or a chlorofluorohydrocarbon group, but this iodine compound is expensive and highly toxic and readily liberates the iodine atoms bonded to the fluorine-containing elastomer, when exposed to light, as disclosed in the following Japanese Patent Application Kokai (Laid-open) No. 60-221,409.
Japanese Patent Application Kokai (Laid-open) No. 60-221, 409, discloses a process using an iodine compound represented by the general formula RI.sub.1-2, wherein R repesents a hydrocarbon group having 1 to 3 carbon atoms, and this iodine compound is cheap and less toxic than the said compound represented by Rf Ix, but is poor in the vulcanization speed, and the heat resistance and compression set of the vulcanized products.
Japanese Patent Applicaiton Kokai (Laid-open) No. 59-20,310 discloses a process using a bromine compound represented by the general formula RBr.sub.x, wherein R represents a saturated aliphatic hydrocarbon group. Inventors of this prior art invention are partly common to those of the prior art invention disclosed in the said Japanese Patent Application Kokai (Laid-open) No. 53-125,491 and state that the fluorine-containing elastomers having bromine atoms are superior in the properties than the said fluorine-containing elastomers having iodine atoms. Indeed, the fluorine-containing elastomers having bromine have a better light stability, but are poor in the vulcanization speed and the heat resistance and compression set of the vulcanized products.
Japanese Patent Publication No. 54-1,585 discloses a process using a brominated olefin compound such as bromotrifluoroethylene, 4-bromo-3,3,4,4-tetrafluorobutene-1, etc., but the thus obtained fluorine-containing elastomers are liable to undergo gelation and are not only poor in the processability (flow characteristics), but also unsatisfactory in the elongation and compression set of the vulcanized products.
Japanese Patent Application Kokai (Laid-open) No. 60-195,113 discloses a process for copolymerizing not more than about 5% by mole, preferably 0.1 to 1.5% by mole, of a vinyl ether monomer acting as a curing site, represented by the general formula: EQU ROCX.dbd.CYZ
wherein one or two of X, Y and Z is selected from bromine and iodine atoms, the remainder being hydrogen, fluorine or chlorine atom, and R is a linear or cyclic alkyl group, or alkenyl group or allyl group.
As the monomer having the curable site of vinyl ether, a compound substituted with both bromine and iodine can be selected from the compounds represented by the said general formula, but the said Japanese Patent Application Kokai (Laid-open) No. 60-195,113 discloses that bromine or iodine-containing vinyl ethers, where the halogen substituents represented by X, Y and Z are bromine or iodine, though a little poor, can be used as the monomer having the curable site. The process also has the same disadvantages as those of the art disclosed in the said Japanese Patent Publication No. 54-1585.
The present inventors have made extensive studies of a process for producing a peroxide-vulcanizable, fluorine-containing elastomer having distinguished processability, vulcanization characteristics and physical properties of vulcanized products to overcome the problems of the said prior art, and have found that the problems can be solved by polymerizing a fluorine-containing olefin in the presence of an iodine and bromine-containing compound, thereby introducing both iodine and bromine as cross-linking points into the fluorine-containing elastomer molecules at the same time.